1. Field of the Invention
The present invention relates to methods for driving liquid crystal display devices. In particular, the present invention relates to a field sequential liquid crystal display device capable of image capture by measure of the amount of light which is used for displaying images and reflected by an object to be read.
2. Description of the Related Art
As display methods of liquid crystal display devices, a color filter method and a field sequential method are known. In a color-filter liquid crystal display device, a plurality of subpixels which have color filters for transmitting only light with wavelengths of given colors (e.g., red (R), green (G), and blue (B)) are provided in each pixel. A desired color is expressed by control of transmission of white light in each subpixel and mixture of a plurality of colors in each pixel. In contrast, in a field-sequential liquid crystal display device, a plurality of light sources that emit light of different colors (e.g., red (R), green (G), and blue (B)) are provided. A desired color is expressed by repeatedly blinking each of the plurality of light sources and controlling transmission of light of each color in each pixel. In other words, a color filter method is a method in which a desired color is expressed by division of the area of one pixel among given colors, and a field sequential method is a method in which a desired color is expressed by division of a display period among given colors.
The field-sequential liquid crystal display device has the following advantages over the color-filter liquid crystal display device. First, in the field-sequential liquid crystal display device, it is not necessary to provide subpixels in each pixel. Thus, the aperture ratio can be improved or the number of pixels can be increased. Further, in the field-sequential liquid crystal display device, it is not necessary to provide color filters. That is, light loss caused by light absorption in the color filters does not occur. Therefore, transmittance can be improved and power consumption can be reduced.
Field-sequential liquid crystal display devices whose pixel portions are capable of not only displaying images but also image capture have also been developed. For example, Patent Document 1 discloses a liquid crystal display device capable of displaying images by a field sequential method and taking color images by measure of the amount of light reflected by an object to be read.